Opacification of resins



condition a quantity Patented Apr. 15, 1941 OPACIFIOATION OF RESINS Glen M. Kuettel, Bloomfield, N. 1., minor to E. .I. du Pont de Nemours a Comp ny, Wilming- No Drawing.

M, a corporation of Delaware Application September 6, 1939, Serial No. 293.56!

Claims. (01.18-55) This invention relates to the opaciflcation of resins'and, more particularly, to the production of whiteopaciflcation in ethenoid resins such as polymethyl methacrylate, both in cast and in molded form. i White opaciflcation of resins such as polymethyl methacrylate has heretofore been accomplished by the incorporation of suitable pigments, such aszinc oxide and titanium dioxide, either with the resin-forming material prior to its polymerization or with the fully polymerized resin. In either case. special care and precautions have been found then subjecting the plastic, thus moistened, to heat and pressure in a mold. The opaciflcation thereby produced is of uniform character, as

viewed by the unaided eye, provided that the comminuted plastic has been not coarser than about 40 mesh and has been uniformly moistened.

Coloration may be eifected by the use of an aqueous solution of a dyestufl, in place of water alone.

Also, in accordance with the invention, white .opacincation is produced in the superficial layers necessary to ensure uniformity of opacification of 7 articles molded fromqsuch resin in comminuted form. The opaclflcation of such resinous material in cast form has, moreover, presented a special problemin view of the difliculty of avoiding aggregation and precipitation of the pigmenting ingredient. Nor has the prior art oflered a feasible method whereby such resins in massive cast form could be opacifled through only part of their I thickness.

is facilitated by the presence in the plastic, as- .sociated with the resin, of plasticizer or ethenoid It is an object of the present invention to provide a novel and improved method of opacifying plastics comprising ethenoid resins such as polymethyl methacrylate.

More particularly, it is an object of the invention to eflect such opaciiication in simple and economical manner without "the introduction of conventional pigments into the material either-before or after its polymerization. More specific objects of the invention are to effect, in articles molded'from such plastics,

a permanent white opaciflcation which may, if

desired, be of uniform appearance, and to effect, in such plastics in massive form, a permanent white opaciflcation of the surface layers. A further object is a process of eflecting coloration oil such plastics in conjunction with such opaciflcation. Other objects will appear from the description of the invention given hereinafter.

In accordance with the present invention, the above objects are accomplished by heating a plastic comprising an ethenoid resin from the group consisting of polystyrene and polymethyl methacrylate, in contact with water at a temperature of at least about 100 C., i. e., with water at or near its boiling point under atmospheric pressure or under higher pressure, or with cient time to eiie'ct the desired opacification.

' It has been discovered that a molded article characterized by white opaciflcation may be made by incorporating with the plastic in finely divided of .water, preferably of the order of from 1 to 5 per cent of its weight, and

of such a plastic in'massive form, e. g., a cast resinor a molded article, by subjectingit to direct contact with water at not less than approximately its boiling temperature. The opaciflcation monomer; thezuse of temperatures substantially above C., and of pressures above atmospheric,

becomes necessary or desirable as the content of such auxiliary substance is reduced.

The term opaciflcation is used herein to include any markedobstruction to the passage of light, comparable in its effect upon the appearance of the plastic to that of incorporating in the plastic mineral pigments, such as titanium dioxide, or pulverized bleached cellulose. The use of the term is not limited herein to the production of complete opacity, i. e., the complete obstruction of the passage of light; it is to be understood as meaning equally well the partial obstruction of light which results in an appearance of translucency. Actually, the difference between complete opacity and translucency (incomplete opacity) is one of degree only. Furthermore, a material which is completely opaque in a thick section may be translucent in-a thin section.

The invention is particularly applicable to plastics comprising polymethyl methacrylate, which, alone or in admixture with suitable modifying ingredients;-may be molded, by ordinary thermoplastic technique, either by compression or by in jection, to produce molded articles of desirabie characteristics well known to the trade, and which is commercially available also in cast form. It is,

however, applicable to plastics comprising polystyrene resins.

Such plastics may comprise nothing but a resin {resulting from complete polymerization of the are characterized in general by a high degree of resistance to water and it is accordingly surprising that, upon being submitted to the process of the present invention, they should develop a cloudiness or opacity, probably the result of absorption of water, as though by a precipitation in manner analogous to that familiar in the cases of water-sensitive plastics such as cellulose acetate. Furthermore, it is surprising that the opacification thus produced is permanent underordinary conditions of exposure and even under prolonged storage at elevated temperature. This opacification does not disappear upon aging nor upon being subjected to conditions promoting the removal of moisture, as does the cloudiness induced by penetration of moisture into a plastic of the water-sensitive type such as cellulose acetate.

Because an article molded from coarse particles of polymer moistened with water is usually spotty and irregular in its opaciflcation and the opacification produced by the invention in a mass of cast resin is usually limited to the superficial layers thereof, it appears that the depth of penetration of the water, which penetration is believed to be the cause of the opaciflcation, is shallow under ordinary conditions of conduct of the invention. As a result of the shallowness of this assumed penetration, it has been found that a uniformly white opacification in a molded article is obtained by the method of the invention usually only when the particles of resin, from which the molded article has been formed, have not been coarser than about 40 mesh and have been moistened with water in a uniform manner.

It will be evident that the processes applied respectively to molding compounds and to masperature applied in the molding of such resins is, as is well known, always well above this point, usually not less than about 130 C. and frequently 150 C. or higher. I

Assuming an adequate supply of water in contact with the surface of the resin, the rate of development of the opaciflcation is a function of the duration and temperature and pressure of the opacifying treatment. It is believed that the rapidity of the development of opacity in the molding of moist granular resinwhich development is complete within the few minutes of the ordinary molding cycle-and the comparative slowness of its development in massive resin immersed in water at 100 C., or slightly higher, is explainable as the result of the influence of temperature and pressure.

In the opacification of massive shapes, the result is most readily controlled by the duration of the treatment. In the molding of moist granular resin, however, the development of opacity is too rapid for control by duration of heating, and, furthermore, the duration of heating in the mold will normally be governed by the molding characteristics of the resin and the design of the mold; here the control of opacity is achieved by the proper proportioning of the water mixed with the molding compound.

The following examples illustrate preferred specific embodiments of the invention. Examples I to V1, inclusive, illustrate the invention as applied to the preparation of molding powder and the molding therefrom of articles character ized by white opaciflcatlon. It is optional whether the water be added to a previously dried granular or pulverulent polymer, or whether such polymer, produced in an aqueous vehicle, be dried incompletely so as to leave the requisite percentage of water. In either case, it is evident that uniformity of distribution of the water will be facilitated by suitably stirring the batch. All parts are given by weight unless otherwise stated:

Example I.--Methyl methacrylate is polymerized under conditions which yield the polymeric methyl methacrylate in the form of granules for the most part finer than 40 mesh. This granular polymer, after having been dried, is screened through a 40 mesh sieve. The material passing the 40 mesh sieve is thoroughly stirred with 2.0 per cent of its own weight of water.

Upon being molded into articles by ordinary technique, this molding powder gives a product of a uniform pure white color, and the molded articles are substantially opaque in thicknesses from about 1*; inch upwards.

Example II.--A similarly prepared mixture of Parts Polymethyl methaciylate (passing 40 mesh screen) 100 Water 1. 5

gives a molded article of a handsome uniform translucent white.

Example [IL-Polymethyl methacrylate pre- I pared by polymerizing an emulsion of methyl methacrylate in an aqueous vehicle is rinsed and dried by methods known to the art. Upon being Example V.-Comminuted p o l y s t y r e n e is mixed with 2.0 per cent of its weight of water and molded while moist. The resulting article is translucent in thickness 0.2 inch.

A more marked opaciflcation is obtained by using a larger percentage of water.

The white opaciflcation may be combined with coloration of the polymer by the simple procedure of dissolving a suitable proportion of a water-soluble dyestuii' in the water used, as indicated in Example VI.

Example VI.Polymethyl methacrylate passing a 60 mesh screen, parts, is thoroughly stirred with water, 2 parts, containing in solution 0.01 part of National Cloth Red 3, a watersoluble dyestufl. (Color Index No. 262). Articles molded from the resulting mixture are of opaque pink color.

Since obviously the molding powder prepared in accordance with this invention is merely superficially dampened with the requisite amount of water, it will be evident that reasonable precautions must be taken to avoid failure through evaporation of this water prior to the use of the granular material. Shipment and storage in airform.

aasauo for 1.5 hours. This treatment renders the block translucent in appearance.

Example VII.A rod of polymethyl methacrylate formed by the process of C. M. Fields. United States Patent No. 2,057,674 is placed in a container adapted to be sealed against internal p essure, and provided with intake and discharge pipes and jacketed for heating and cooling. I The container and contents are then heated by the e of boiling water through the container and the introduction of steam, under a pressure of 60-70 pounds per square inch, into the jacket. When the rod has been brought to a temperature of about 100 (1., the discharge valve of the container is closed and the supply of boiling water out off, and the intake valve is then connected with a source of hydraulic pressure of 2500 pounds per square inch. The intake valve is left open and the steam circulation in the Jacket is continued for 90 minutes. The steam circulation in the jacket is then discontinued and cold water circulated through the jacket for about 30 min-' utes; thereafter, the hydraulic-pressure is out off, the container opened, and the rod removed.

If the rod has lost its straightness by this treatment, it is immersed in boiling water and then rolled on a flat surface before it regains its ri idity.

The rod thus When the rod is cut, it is found that the opaciflcation is localizedat and close to the surfaces which have been in contact with the hot water in the cylinder while the main body of th rodretains its original eolorless.transparency. By suitable carving, beveling, and the like, the opacified layer may be cut away, revealing the transparent body material beneath. Invthis manner the rod may be fabricated into umbrella handles, cutlery handles, et cetera, of ornamental character. I

Example VIM-A .rod'of polymethyl methacrylate having a hexagonal cross-section is formed by the process of C. M. Fields United States PatentNo. 2,057,074 in a mold of hexagonal cross-section. This rod is treated as in Example VII. Its six dihedral angles are then beveled oil. so that it is converted into a rod of duodecagonal cross-section of aitemate white andtransparent faces. i

Example IX.-By the method of C. M. Fields United States Patent No. 2,057,674 there is formed a rod of polymethyl methacrylate of pink color, this coloration being provided by the addition of 011 Red (Color Index No. 248) 0.01 part, to the monomer, 100 parts. This rod is superficially opacifled by the method of Example VII. It thus becomes of a translucent pink as a result obtained is of a translucent white.

It will be understood that the above examples are merely illustrative of the invention' which broadly resides in opacifying an ethenoid resin from the group consisting of polymethyl methaerylate and polystyrene by holding the resin in contact with water at a temperature of at least about 100 C. for a period suiiicient to effect the desired opacification.

Although the invention is broadly applicable to the polymethyl methacrylate and polystyrene resin plastics, including interpolymers and mixtures of such resins, some consideration must be given to the physical properties of the plastic to determine if the treatment of the invention is prac tical. This is chiefly true where superficial opaciflcation of a molded article is desired. For example, if the molded article is made from a plastic which tends 'to be deformed or to an-- mum conditions for obtaining the opacity desired in specific instances, the factors influencing the speed and/or degree of opaciflcation having been discussedabove. 1

The present invention offers the advantages of a simple and inexpensive method of developing white opaciflcation in molded and cast ethenoid resins. Opaque and translucent whites and tints of the illumination. of the white translucent surcan be obtained by simple treatment of an unpigmented polymer and the resins so opacified are adapted to practical and ornamental uses.

The invention also provides a simple method of opacifying the exterior layers, only, of massive plastics, which can then be carved, beveled, et cetera, for the production of ornamental effects.

The opaciiication produced is pure white. Color can be added by the use of dyestuffs. As many apparently widely different embodiments of this invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as deflned'in the appended claims.

I claim:

i.' Process of permanently opacifying a plastic comprising an ethenoid resin selected from the group consisting of polymethyl methacrylate and polystyrenewhich comprises holding said plastic in contact with water at a temperature of at least about C. for a period suiilcient to effect the desired opaciflcation,

2. Process of permanently opacifying a plastic comprising an ethenoid resin selected from the' group consisting of polymethyl methacrylate and polystyrene which comprises holdingsaid plastic in contact with'water at a temperature of at l'east'about 100 C. and under superatmospheric pressure for h period sufficient to effect the desired opaciflcation.

3. Process of producing a molded article characterized by opaciflcation, from a plastic comprising an ethenoid resin selected from the group consisting of polymethyl methacrylate and polystyrene, which comprises moistening said plastic mersed in water boiling at. atmospheric pressure 7 in comminuted form with water, and then molding the moistened plastic under heat and pressure.

4. Process of producing a molded article characterized by a uniform opaciflcation, from a plastic comprising an ethenoid resin selected from the group consisting of polymethyl methacrylate and polystyrene, which comprises moistening said plastic in comminuted form free from particles coarser than about 40 mesh, with 1-5%, by weight thereof, of water, and then molding the moistened 10 plastlcunder heat and pressure.

5. Process of permanently opaciiying a molded article comprising an ethenoid resin selected from the group consisting or polymethyl methacrylate and polystyrene which comprises holding said molded article in contact with water at a. temperature of at least about 100 C. and under superatmospheric pressure tor a period suflicient to effect the desired opaciflcation.

GLEN M. KUEI'I'EL. 

